Everyone's saying to ask for money … which is fine, but also don't forget to be human and make/keep friends. If someone's asking for a little bit of help and you're feeling up for it, by all means help them (and don't feel like you have to bill for the help to avoid being used). If you're not feeling up for it, you can decline ("sorry, I'm busy/I can't fit this into my head right now"). If it's a bigger ask (e.g. traveling to them for a longer engagement), then yes absolutely agree on compensation ahead of time.