I'm arriving in terminal 3 ahead of my friend who I'm meeting at terminal 5 Costa Coffee shop for our ride to our hotel. How do I get from T3 to T5 arrivals?
The Heathrow website has a wealth of information
https://www.heathrow.com/at-the-airport/airport-maps/travel-between-terminals
simple answer, you'll be given a choice of a free tube trip, a bus trip, a free Heathrow Express or Elizabeth Line trip connecting T2/T3 to T5.
I personally would prefer the free Heathrow Express or Elizabeth Line (whichever comes first on the same track) for the quick one stop ride. When you get to T5 go upstairs and follow signs to arrivals and find the Costa.