When a colleague celebrates his birthday, he usually invites his colleagues to go together, which can be regarded as a way to bring the relationship closer. Since he has been invited, it is better to go. After all, working together will make the relationship smoother. As for gifts, you have to give one no matter it is out of affection or courtesy. I can give you some small suggestions:
1. Since she is a female colleague, she should be very concerned about her office environment, so you can give her a cartoon ornament, which is also very good to put on your desk.
2. You can also give her a color-changing cup with her photo on it. When you pour hot water into it, her photo will appear. It is a very interesting gift.
3. You can also give her a big shy bear. Girls will like such gifts very much. Cute things at home will also make her feel good.