Where is customs in Heathrow?

Both terminals are separate, in different parts of the airport. They are effectively different airports. When you land the sequence is:

• Disembark, walk through corridors
• Immigration (passport control). This is the slowest part, 20-60 minutes.
• Luggage reclaim
• Customs. This is a walk through with random checks only. 95%+ do not get stopped.
• Exit into arrivals area. This is a public area and is full of people waiting for people arriving.

To get from terminal 3 to terminal 5 you then need to catch a train. Info here from official Heathrow site: https://www.heathrow.com/airport-guide/getting-around-heathrow/travel-between-terminals

Terminal 3 to Terminal 5
Heathrow Express trains are free for travel between Heathrow Central station (Terminals 2 & 3) and Terminal 5. There's a train every 15 minutes.
Average journey time: 20 minutes

In view of the difficulty of meeting up, i would suggest both making your way to your hotel independently, and meeting there.

"In view of the difficulty of meeting up, i would suggest both making your way to your hotel independently, and meeting there."

Yes, this. ^ ^ Even if you do have a plan to meet up, be sure that each has the ability to get public transport to your hotel. Planes can be late or early and it may be hard to communicate if one is still flying!

Technically it is possible in most cases to clear immigration in a different terminal from the one directly arrived at by using airside buses if you don't have checked bags. However, this doesn't really help you as it is just one more variable in timing.

My friend and I are not staying in London, so making out way separately to where we're staying isn't a viable option. Also since we both have booked luggage we will need to go through customs at the terminals we fly into. I called British Airways after posting here to ask if they knew (since they do fly into both terminal 3 & 5). Their helpful answer was to ask someone at the airport!

If you are not staying in London, where are you heading for and by what mode of transport?
That might suggest a logical place to meet up (car hire office, bus stop).

Or are you in fact going into London and immediately out again? If so, Paddington station is a logical place to meet up.

I would also ensure you both have working phones. So you can communicate if it all goes pear shaped. That would imply not buying a new SIM at Heathrow, new SIM = new phone number.

Chris, your detailed answer in your first post was especially helpful. Thank you.

As I'm mostly just familiar with terminal 5, I checked Heathrow's website for locations for their "Meeting Place" signs. I notice them everytime I leave the secure area, but couldn't be specific as to their location.

Here are your options per the website: Where to meet/ Meeting point icon
The official airport meeting points at Heathrow are marked with signs, and can be found in the following locations:
Terminal 2: Opposite the exit to international arrivals
Terminal 3: Arrivals building, ground floor seating area
Terminal 4: Next to WHSmith in arrivals (ground floor)
Terminal 5: There are two meeting points on the ground floor, marked by banners:
North meeting point: near green staircase, opposite Travelex
South meeting point: near orange staircase, near Costa.

However, I might recommend scanning nearby one of the above meeting places to see if there is a coffee shop so one could wait while fueling.

A couple years ago my friend flew into Heathrow from Seattle, arrived at Terminal 5, I flew in from Ohio arrived at Terminal 3. She took the train to Terminal 3, she was there waiting for me when I came out. Had no problems!!!