Another dilemma

So fast forward from my travelling problems , which yes I could go on & on about but I won’t .

Next month is a big birthday for my DH & DD2 & SIL are giving him a surprise party at their house. Garden weather permitting 🥵
Not a big bash but we have 8 children between us & numerous GC & now GCC .Its going to be a surprise & as it’s at their home he will have no idea . So far so good .
It’s just a family event in the garden nothing major celebrating a big birthday & as my DC all say he treats every one & they want to treat him .
On to the rest of the guest list , a few of his good friends & my DH’s ex wife who we /I get on with spend their 2 DS birthdays with etc . However on calling her to invite her she asked if she could bring her “New BF”
So what’s with my dilemma if we get on so well ?
Because I feel she’s making it about her .
She hasn’t known him long matter of weeks, he will be an unknown quantity to everyone else there .
I feel a bit mean saying “No” but that’s how I feel .
,I said I’d check with my DD & SIL
but it’s obviously also my decision as well . It was also my GDs who asked if I was inviting her & we made that decision as a family when the whole party idea came up
.
Opinions please 🙏