Be happy for her. Simple as that. But, if she’s changing into a negative version of herself, then there is cause for concern.
No matter who she is in a relationship with, he is going to impact her, at least a little bit. If she is just changing into a better, more positive version of herself, you need to be happy for her. Even if this means that she won’t go to the bars anymore, because she hasn’t enjoyed drinking ever since she met him. Even if it means she doesn’t want to go to the frat parties anymore, because he can’t get in. Even if you two can’t share hookup stories anymore, because she met someone who doesn’t want to leave her side. Even if it means she can’t spend every second with you anymore, because she has decided to make room for someone else.
If he abuses her or makes her sad or won’t allow her to spend any time with you anymore, then you should be very upset with her… but if she is in a healthy, happy relationship, then you should be happy for her. It can be so hard to share your best friend with a new guy, but she needs you to stick around and be there for her.
At some point in all of our lives, someone special is going to walk in and turn everything upside down. You need to be happy for your friend because of the beautiful mess she is in. Her world became something entirely different, and she loves every second of it. Her being in a relationship doesn’t mean you aren’t part of the picture anymore, now she just has two frames.
A new boy is going to influence your friend no matter what. If the changes make her a happier, better person, then try not to be bitter –especially if you want a wedding invitation.