Condoms and pills can fail. Also she is in a long term relationship and most couples like that wouldn't want to use condoms. Also re pills, if contraception is no longer covered by affordable care act they could turn expensive very quickly. This is America where hundreds can be charged for essential life saving drugs. Drug prices can also change and spiral due to shortage issues.
I mean it's not the same but my dh has spent hundreds on adhd medication as he gets his prescription privately. He got his diagnosis privately so will do private prescription before shared care is agreed with GP. This is what medical care costs and if a one off procedure can take care of it forever and you are sure why not.
For me if I did the iud and it was 10 years (hate the idea of something in me) I don't like to think how much it would cost to remove and replace it in 10 years time privately as I suspect nhs appointments would be few and far between or downright available. 300 quid now so would be double that or likely closer to 1k (maybe thats a lot of money to me then). Right now they are charging over 2k privately for a vasectomy at my dh's usual private care provider so he decided to go for nhs vasectomy, he usually doesn't use nhs and vasectomy is a pretty simple procedure.