Whilst extra curricular activities are important, particularly swimming lessons, contact time with dad is also important, if not more so. As it is he doesn''t get to see them much, and a lot of his time with them is, as you say, cut short by the swimming lessons, and not good quality time because of all the travelling to take them to these lessons.
It is therefore reasonable that mum should accept a compromise in this situation. I think your partner collecting the girls on a Friday evening and returning them on the Sunday evening every other weekend is perfectly acceptable, and is in fact considered to be very normal contact, and what most NRP''s enjoy, along with half of all holidays and alternate Christmases etc.
If all your partner is getting in terms of contact is 1 night per fortnight, then this isn''t enough, and add to that the swimming lessons, and it''s less than satisfactory.
I think he has two options here. He could write to mum formally, saying he doesn''t wish for the girls to give up their swimming lessons, but as it is, and as he is expected to take them to these lessons, his contact time is cut short and so he therefore wants to have them from Friday to Sunday to have more quality time with them (he could add that he wishes to take them out for the day, or visit their extended family). Or, if he hasn''t got a Contact Order and mum doesn''t agree to any changes to the current regime, he asks her to attend
mediation to try and sort this out and then, if unresolved, applies to the Court for a contact order. As it goes, the Courts take a very dim view of activities being arranged, seemingly deliberately, during the NRP''s contact time.