Is it legal to work multiple jobs on H1-B Visa

My friend is in US on H1B visa.

Can he work multiple jobs on H1-B Visa.

Does it depend on visa type or the employer.

Does he need to notify his present employer or seek any NOC from them to do so?

you can have multiple H1Bs but not work for them.

You can work for only 1 at a time.

If however you have multiple part time H1Bs, I believe you can work/