Heathrow does have some meeting points, but most of these are aimed at meeting arriving passengers landside. I'd suggest you avoid those, as otherwise you'll have to clear immigration which you don't really want to do if you can help it.
For both of you, when you land, follow the (largely) purple Flight Connections signs. You can see an example of what to look for in this answer on a T5-T5 connection - it's largely the same signs in all the terminals. Flight Connections will eventually lead you to somewhere where you can take a bus to Terminal 5, make sure you get the right one as it's a different bus for each terminal! Pretty well sign-posted though.
On arrival at Terminal 5, the bus will drop you off at the ground level, and you'll take the lifts or escalators up. Follow Flight Connections here, and you should see this:
If you still need your onward boarding card, head to the connections desk on your right. Armed with your boarding card, straight on to the connections boarding card check, then upstairs for transfer security. Chances are, you're looking at about 45-60 minutes from stepping off your plane in T1/T2/T3 until you've cleared T5 security, with walking, wait for buses, and security queues. Times will vary for both of you, so it's hard to say who'll be first!
At this point, you have two options. One is head straight to your gate in T5, and wait there for your friend. There's seating at every gate, and the first one there can have a seat while you await the other. There's free wifi in the terminal, IIRC after watching an advert, so you can get online while you're there.
Otherwise, there's a few restaurants and bars in terminal 5, listed here and shown on this printable map. You can always pick one, and whoever gets there first gets the first round of drinks in, the other finds them in there on their arrival!