oh god agreed what pressure for a child.
I think out of respect for the other partner you do need to say if another adult will be involved in the childs life, however there's no need to make a big issue out of things. I'm lucky in the sense I don't see the GF often, but am quite happy for DS to talk about his time there etc & tbh am happy & glad that he enjoys his time with his dad/GF.
from this POV, it's not a major thing for her/I to be good friends, there's no malice there, feel sorry for her with XH as she's very young & slightly nieve I think but ah well she's young she'll learn lol. key things that both XH & I agree on is we respect each other as the other parent, if DS's behavior needs discussing we both talk to respective partners & then talk to each other.
but ultimatly I don't worry because DS is happy. if DS was in any way unhappy/worried/anxious I would have made a big point about meeting the GF, spending time getting to know her etc etc but it's honestly not an issue. I wouldn't say we'd ever sit & have a coffee just us girls in the cafe but likewise if she's a permeant fixture in DS's life then there needs to be openess & some sort of relationship.
Likewise XH is the same with DP, whom he's met more as DP is usually around when XH brings DS back. Just keep things casual, expect a few teething problems and be open & honest you'll do fine. :). FWIW i've not met DSS's mum yet after a year, not sure if I will do, I suspect I will at some point, will be slightly odd, however she also trusts DP with her child in turn knows he wouldn't introduce his lad to a random stranger so it's not been necessary. I suspect if DSS requested it then we'd all meet up and get along for a bit.
I find as well, with DSS when he's with us I came along thinking I'm a friend, another adult he can talk to & be silly with/do homework with, not at any moment have I thought right i'm this child's step mum, he's my responsibility & I need to be the mother when he's with us. (usually do unconciously but that's my maternal side fussing lol)