Look, the answer is Yes, he can have a girlfriend.
This fact will not generally affect his legal position and assets (if a Court is left to decide). Generally parties are encouraged to move on with their lives. It can even be seen as a positive thing.
But if he is trying to reach a Binding Settlement with his ex-wife, (outside of Court) - common sense is that it might cause her to walk away from the negotiating table, or negotiate harder out of spite.
Assuming a Court is left to decide... there is a caution, that comes with the above advice.
The caution, is that if he goes about the whole thing, in the wrong way... and it creates a whole mess at home... and kids are involved... or he rubs his ex-wife's face in it... etc. then his conduct could count against him, if a Court is left to decide everything. One example, it that he could be asked to leave the marital home. If he wants to leave anyway, that might be no teal issue.
If he handles it discreetly and privately (at least until the Binding Deed of Settlement) is signed... that would be the most prudent course of action.
In the end many other factors also contribute and affect any judgement by a Court in property settlement and custody matters... so, my caution, is just a caution so that he does not take the permission to have a girlfriend, as a licence to cause problems for no reason.