Brian,

At the 52,500 service on my 87 124ce/103 2 years ago, plug inspection revealed oil on a couple of plugs. At that time I was using a qt every 1500 miles or so. I was told I would probably soon need valve seals. At 56,000 a few months later, comsumption worsened (800 miles).
I had the job done (around $400.00- parts 36.00, the rest labor). End of problem. Now, with 74500 on the clock, plugs clean and oil consumption normal, which is to say no need to top off between oil change intervals 3-3500 miles.

If your friend is consuming more than a qt. every 3000 miles or so, this is a possible (even likely) suspect.
Valve seals and Valve guide wear (a bigger repair) were common problems on the 103. There are many past posts on these issues.

Good luck.