Boyfriend and his friend.

You're paranoid. Both people are in relationships, and there is literally nothing to base a suspicion on besides they spent a day together and he's staying at hers.
Spending a day together is not unusual between friends regardless of gender and staying at hers, well just talk to him about it if you really must.
How long have the relationships been on, yours and hers? Is your relationship experiencing problems and does your boyfriend have a history of cheating?
Be careful about this. Its easy to feel insecure in these sorts of situations, but that is why you need to trust your boyfriend to not do anything.

Theyve known each other since September because they're on the same course. And also he needed a place to stay and errands to do during the time. The thing is, i know he'll probably spend more time because he'll be back in September before me and she's one of the only people who live there. And he doesn't like to be on his own.

But i do feel like hes spending a lot of time but, havent seen her since June.

What do you mean by socially? Its pretty much like, he was spending 2 nights because of some errands and spending time with her and needed a place to stay , so she offered him her place since she lives there. And he walked her to work at 10 pm and back at 3am because its far and was dark

Theyve known each other since September because they're on the same course. And also he needed a place to stay and errands to do during the time. The thing is, i know he'll probably spend more time because he'll be back in September before me and she's one of the only people who live there. And he doesn't like to be on his own. But i do feel like hes spending a lot of time but, havent seen her since June.

These all seem like fairly legit reasons for him to be staying with her and spending time with her.*

If it bothers you, just have a conversation with him about the fact it's playing on your mind. With this additional info it sounds like there is nothing wrong with what's going on, so just speak to him and put your mind at ease.

Considering he had good reasons to stay at her house, I think you're pretty paranoid. Imagine if she was a guy, would you be ok with them spending so much time together, especially when nobody else is around for him to see (including you)?You can talk to him about it if you're worried, but you shouldn't need to. Trust is important in a relationship and the gender of his friends shouldn't matter.

It depends on the context really. I have female friends who I'll spend the day with and stay the night absolutely just as friends, but that's because I no longer live near them. If it was someone who lived pretty close, it would be at least unusual.

Ive done that . And left him a voice note so ai can fully explain. But what about during the ending of the month to 10th of September? Should i trust that he'll listen to my concern?

Find yourself another boyfriend, because this one's gone.

Sorry. You can listen to all these other people here feeding you a delusion, but he's gone.

The only place this works out ok, and they're just good mates is on an internet forum. In real life, they're either doing it or they're just trying to figure out how to rationalise it.

My advice is - don't have any drama or diva stuff. Just tell him you're done, because that's how it is. You're not having any of this besties business. Don't give him a second chance. Just move on. If he calls up or texts - just say you're not having it. No need to be a drama queen - say he's done this, and that's all there is to it.

Great advice - when you don't like something, throw up your hands and run away.*

It's walking away with some self-respect.

What's the alternative, a long drawn out string of increasingly whiny texts, a summit meeting for a "grown up conversation" and then a lot of excuses, guilting and counter-guilting?

What this guy is doing here is blatant, and it's obvious that the OP isn't ok with it. If he has loads of female friends - no big deal. If he goes out with them all the time - fine. If they get drunk one time and they make out - that happens.

But what he's doing here is sidelining his girlfriend openly. This isn't a drunken happening, this isn't flirting. This is going over to some girls house for effectively 3 days. He's blatantly telling OP he doesn't care what she thinks.

Any conversation OP has, or tries to have, she'll be painted as the insecure girlfriend. Don't have that. Don't give warnings or second chances on this kind of thing - get out with dignity and leave him to it.

Most other situations, I'd say fine, you can talk about this. If he'd come and said he'd slept with some other girl, it was wrong and he's sorry - that's totes different. Then OP can decide what she wants to do. This is a situation where the guy is just having a laugh at her expense.

I'm not sure he is blatantly telling OP he doesn't care what she thinks, because until halfway through this thread she hadn't told him what she thought.*

There doesn't need to be a string of whiny texts; she's now communicated her feelings. He might decide to stop spending so much time with his female friends in this way - if so, then that is him going out of his way to accommodate OP, because on the face of it I don't think he's doing anything wrong here. If he doesn't, and OP can't handle him staying at a female friend's place (even out of convenience or necessity), then that's her issue and she can decide what to do at that point. Advising her to skip immediately to ending it without so much as a discussion strikes me as impulsive and a bit immature, but I may be in the minority.*

OP, this is the simple answer

She's not in a relationship anymore and mine has been 9 months . And its fine to spend time but its the nights i worry about if im honest but i hear you