That is very confusing. You want to see your ex before he joins up, but talking about visiting during 1 week off in training?
Actually, that's not correct anyway. Depending on how old he is and where he's training, the first few weeks are very intense and communications strictly limited, but then it starts to relax, there are weekends off and he will have use of laptop, iPad etc.
Dors your ex know your best friend well enough to have her around or will she play third wheel? And he will have a lot to sort out with packing as well as saying goodbye to people, so I would at least check with his sister; however, make sure she is clear on his plans rather than just turn up if she says "I think he's around."