I don’t think about my daughter’s friendships in terms of whether they are ‘stretching’ her in any particular way - what a strange way to think! That’s not what friendship is for, and this mum sounds like she doesn’t really understand that and is in danger of alienating her daughter from a friendship that she very much needs (as all children do).
In many years of teaching (including in specialist SEN provision) I never really thought about friendship in terms of whether X was ‘stretching’ Y either - that’s the job of the teaching staff which can be done in a number of different ways including in pairing or grouping children in different combinations for different tasks. The idea of trying to interrupt a friendship or separate friends who are otherwise good for each other merely to provide the other with a bit more academic challenge is ridiculous.
The mum was incredibly insensitive to say this to you - it sounds like she thinks her daughter is the more able of the two and that in a perfect world she’d prefer a friend she perceives as being on the same level, but since that’s not available she’ll settle for what she’s got and continue to allow the friendship and allow them to stay in the same class. And that she thinks you should somehow be grateful about that. It shows a lack of tact and fundamentally a lack of understanding on the mother’s part - that she thinks because her daughter is more talkative that means she’s doing better overall.
Honestly I’d continue to support your son in any way you think would benefit him, including in this friendship, but maybe take a step back from the mum if you can.
If she brings this stuff up again the teacher in me wants to say it’s just her projecting her own insecurities and worries about her DD, and to just say, ‘that’s such a strange way to look at friendship, it’s not DS’s role to stretch your DD silly, that’s what the teachers are for! It’s lovely they are so close’.
The protective mum in me, if it was my DD she was talking about, wouldn’t be able to help saying something like ‘yes I had my concerns too if I’m honest, still I’m glad your DD can continue to benefit from being friends with someone like DS who is so calm and kind, he’s such a great example for her in terms of emotional regulation don't you think? I definitely agree with you that they’d both benefit from maybe widening their circle a bit too. Hopefully we can work on that next year’.