Upvote:2
Provided your friend is also a comfortable traveler, the easiest thing for you to do is take the Underground (Piccadilly Line, one stop) from T5 to T2&3 and wait for them on the platform.
Or, if they manage to arrive first, they can wait for you there. Basically, the T2 platform is your rendezvous point.
If you have a mobiles that works in the UK, you can check each others flights. Heathrow has free WiFi if your plan does't cover text or data. T-Mobile is likely included, depending on you plan, and the other carriers make it easy to add. You really should do this anyway.
Upvote:6
The most convenient meeting point is indeed the T2 public arrivals area. It offers comfort, access to food and drink, wifi, immediate updates if your friend's flight is delayed, the ability to make enquiries about any delay and page her if you don't manage to meet.
Arrivals areas are accessible to anyone, so make your way to T2 by any of the advertized means and simply wait with everyone else who is waiting for a passenger.
There is a page on the Heathrow website dedicated to getting from one terminal to another. In short you can use the free Heathrow Express train, or the Underground, which is free if you have an Oyster card.