As a team lead, she has a managerial position, and at that level one should be expected to manage their own time.
Her situation is nothing special. Yes, it's a bit arbitrary for her boss not to let her work more, but this is in no way different to any other overworked manager (most managers I ever met have more work on their hands than possible to work off in the allotted time if they wanted to do everything perfectly).
So I'll concur with the other answers and suggest she tells her boss about the fact that she has less time than before (he surely knows already, but she may go a step further and tell him exactly how much it is). So far so good. But I'd also expect her to work out a plan of what to remove from her work schedule. Maybe two alternative scenarios if at all possible. Then she needs to document the pros and cons and give her boss either a simple "either/or" decision, or better yet, decide herself and let her boss simply nod it off:
Hey boss, I have come up with plan A or plan B. Which do you prefer?
Hey boss, I have come up with this plan A to get along with less time and will implement that starting next week unless you have more input for me (I could also follow plan B if you prefer that).
Obviously, plan A should be the one she likes more / finds more sensible etc.
A top way to build such a plan is to delegate stuff (which could mean getting more staff or an assistant; or motivating another team lead to take some of her stuff). Next, de-prioritize/skip complete work packages. Next, resolve some work faster, if possible in her job (which may cut into quality, obviously). She also needs to say "no" more for new work entering her domain.
The one thing she must not do is plan in a way that makes her suffer more (i.e., skip lunch, stress herself out overmuch, etc.).
Oh, by the way. A boss who actually wants to avoid overtime should be a luxury. It would be much easier and profitable (at least short-term) for them to keep her working until she dies of exhausting. A boss is responsible for their directs and especially their health and well-being; sure, at 30h/week, 10 hours of overtime will not damage your mothers health - most people work 40h/week easily. But still... the fact that he tries hard to avoid overtime in general is to be applauded. Depending on the size or type of her company, he also simply may not be able to pay more than a 30h/week position - he may well be out of the loop which decides these things.