I apologize in advance if this offends you; that is not my intention, and what I offer here is merely my own perspective (I freely admit to my distorted view of reality), solely based on your original post.

1. Your daughter is using a camera designed a decade ago, for personal, hobbyist use; you don't mention when it was purchased, but that's likely to be more than 5 years ago. It's a fine camera for the average consumer, but it wasn't designed and built to be used as a professional photography tool.

2. As others have already commented above, it may have just failed as a consequence of being worn out, or there's always the possibility of user error (and I hold up my hand, I've had that experience too!). If a local camera shop will take a look at it without charge, they may spot a simple remedy, or at least be able to quote you for the likely costs of a repair? At that point, you may find that repair of this relatively inexpensive camera body isn't financially the best use of money, and/or the downtime, while waiting for the repair?

3. If your daughter needs to urgently complete a paid commission, the most expedient option is probably to buy a replacement camera body. Used T3i's (in good condition with a limited warranty) are available from reputable camera shops for $150-$175; not a bad investment to rescue the commission, and that will also provide a reserve body if the original is cheaply repairable, or it turns out to be a user error? The advantage here is that it's an identical second body, so zero learning curve to experience before completing the commission.

4. I'd recommend for anyone taking their photography from hobbyist to paid commissions, to seriously look at a cost-effective upgrade to a professional build quality body (ideally two, because of the risk of accidents or failures during a paid commission). In my opinion (and again, I freely admit my perspective may not be others), the best value for money Canon body on the used market at the moment is the 7D Mk II; prices start around $600, for a body with years of life left in it. You can also pay more and get a package of body and lens; a cost-effective way to upgrade with better glass, if needed? The advantage with the 7D Mk II is that it was until recently the top of the range Canon crop-sensor body (so your daughter will have no problems with existing lens incompatibility), and has a host of very useful features that will really help her photography (although there will be a small learning period).

I do hope that this advice is helpful; as I don't have the T3i body in front of me, these are the only suggestions I can make. Whatever the outcome, best of luck for your daughter; I hope she can complete her commission in a timely manner, without too much inconvenience and cost.

Thanks to everyone who added insight into this, although it's been almost a year since I asked the question. As a follow up, since I now have the camera in my possession,(she lives in FL and I'm in PA and current conditions made visiting impossible. I ran the camera through some screenings, cleanings and adjustments as suggested. The end result seems to be that the sensors on the 18-55mm lens are bad. I hooked up my 75-300mm and all systems seem to work. Is it possible to replace the sensors or is it best to get a new replacement lens? Thanks again, and Happy new year to all.