Passport control is the same thing as Immigration. This the check on people. Checking whether you are allowed into the country and if so for how long. This is where your passport is stamped if necessary.
Customs is the check on goods. Checking the contents of your luggage to see if it contains any forbidden items or items for which customs duties (taxes) must be paid. Customs is the one you need to worry about the least, as most people are not stopped, they just stop and check a small random sample.
You don't say where she is changing planes, but I guess it is somewhere in Italy.
Yes, she will have to go through immigration (passport control) at this intermediate airport because, as you say, the UK is not part of the Schengen Area.
The big question, is this one through ticket or two separate bookings?
If it is one ticket, her luggage will be labelled through to Brindisi. Then the two hours will be enough (in the airline's opinion), they wouldn't have accepted the booking if they thought it wasn't possible.
There will be no customs as the UK and Italy are both (until March 2019) in the EU.
If it is two separate tickets, 2 hours is probably not enough. She would have to pick up her luggage at the intermediate airport, then check in for the second flight, hand in her luggage and proceed to the next flight. She may well not have enough time for the minimum check-in time required by the second airline.
