I thought it would be useful to understand how both sides can look after the other even though my scenario below will only cover one side since that is my only experience. So to make this answer more useful, happy to have both 'boyfriend looking after girlfriend' and vice versa included in the answer, since there'll be some overlap anyway (so feel free to switch gf with bf in scenario below to make this easier).

Scenario: My girlfriend and I have been together for 5 months and enjoy going to parties together. We both always have a great time and are comfortable doing things together and splitting off to do our own thing and then reconnecting. We have a lot of the same interests which helps. We both drink but (without being insulting) my girlfriend drinks more than I do or at least often ends up more drunk. So the main thing I find challenging is how to look after her without spoiling her fun? Obviously I get worried seeing her drunk and doing things that are probably safe when sober but no so much when not. But at the same time I don't want to act like her dad, so that fine line is hard for me, especially when surrounded by groups of people who are doing the same things she is. A few specifics now:

• What are some good ways to look after her when I can see she is not sober?
• Is it ok or weird for me to take her to the bathroom (at a house party) and stay with her until she's done? Eg. in case she slips or starts to vomit
• What are some good ways to encourage her maybe it's time to go home or to slow down for a bit?
• If a friend(s) of hers say they'll 'look after her' what should that mean for me, since I still feel responsible?
• If things get to a point where I am really concerned but not able to encourage her to leave/slow down what should I do?

We have already talked about the above together and she trusts me enough to do what I think is right to keep her safe (she's fully aware of how 'fun' she can get), it's just that how a person reacts to the above when sober vs not, is different, especially when you are having fun. Happy to add any other details if they're needed.