Shouldn't be a problem - even though they'll have to go through the security process again (although it's quite pain-free at T5). I'm a smoker so I'll always try to find away to get out of the airport for a smoke between flights (sad, I know!). Flew from Dubai to Cork yesterday morning via Heathrow, and had to go from T5 to T1. First thing I did was to go out of T5 as if I was finished my travels. When finished puffing, I just went back through security and took the flight connections route to T1