Still not getting what she's done wrong. She's invited the people she most cares about to her wedding and your DP isnt one of them. She's also invited husbands as plus 1's perhaps not great as an exclusive criteria, but a widely excepted thing to do, then invite partners based on other factors or in her case not at all.
It's not about the 50-100 just for your DP, although thats not insignificant - am sure if it was an informal gathering or night out he'd be invited! But more likely shes done what most of us do, writen a big list of people she'd like there including your dp, then realised shes got more people than she thought, is a thousand or so over budget or have too many people for the venue and needs to cull a few people. If faced with a choice between extended family, work colleagues, school friends, kids, partners or partners of less than x years then most partners and most non family kids are common choices.
Really dont get the upset when you were happy to go without him initially. Or why people are saying she lied, she didnt. She didnt invite partners as plus 1's just, i assume, partners who are close enough to the couple to get an invite in their own right.
Who would you rather she bumped from the guest list and when did it become the done thing to try and dictate a wedding guest list?