I've been in two clinical trials (and am just finishing my time on the last one).
The drugs depend on the trial, so you can't really generalize. There are vastly different types of trials.
In some cases, they are just using known drugs in a different way. For example, in the trial I'm in now, there's a drug known to be very effective for keeping certain kinds of breast cancer from returning in post-menopausal women. I was pre-menopausal when dx with cancer, but chemo threw me into menopause, which is not uncommon. The trial is to see if this drug is effective in this situation. Many oncologists had been prescribing it this way anyway, but the trial will gather solid data to see if it really works in this situation.
The first trial I was in involved adding an additional known drug to a standard chemo protocol to see if it is more effective than the standard treatment alone.
In other cases, the patients get completely new drugs or therapies.
I have been monitored more closely on both trials, which in my situation is something I like very much. Also, the patient is often given the treatment for free in a trial.
I agree, your mom should talk this over thoroughly with doctors.