Membership of the single market isn't something you negotiate.
It's a set of rules that underpin the functioning of the "market": free movement of goods, services, capital and people. It's a "single market" because it represents a domestic market like the market of a nation state. It's only within nation states that goods, services, capital and people can move freely without any regulatory controls.
If you impose regulatory controls on any of those then you're no longer part of the single market. You can still look to sign a trade deal with it as an external partner but you're not participating in it.
Saying we should have negotiated access to the single market is not understanding what the single market is. What you probably mean is, she should try to negotiate a trade deal that gives very preferential access to the UK when it is outside, ie the UK imposes regulatory controls on the free movement of people so it isn't inside the market, but still retains tariff free trade on goods, free movement of capital and services, and is part of all the EU schemes on cross-border enforcement. That is still something that she can try to negotiate although other EU countries will probably not agree with it.
The other side to this is that non-tariff barriers (eg things like regulatory standards) are generally much more significant than actual tariffs in terms of barriers to trade. But in practice, you create a non-tariff barrier the moment you have a different rule than that of the single market. So you can't 'negotiate access' to these, you either say we will copy the EU's product standards rules, which gives businesses certainty that everything they produce for the UK will be up to the standards allowable for sale in the EU, or you say the UK has the right to set its own regulatory standards, which means the moment you start diverging from those rules you have different standards and so have a non-tariff barrier which in practical terms means you need to pass some system of testing and verification before selling to the EU, which will be a cost born by UK firms that want to export to the EU.