This depends on the side project and the type of work you want to get done. All tech? Or tech work with all of the other work that goes with a startup?

If you want full support 50/50. If you plan on doing most things yourself then balance it, 80/20 or 70/30.

If it pays for itself, it would probably be best to keep 100% until the project is more formalized (a company exists). Equity is quite precious and is hard to get back if your teammate does not work out.