Same as any other Marklin loco, I think - you'll need a 5-pole motor conversion kit, a DCC decoder, and some soldering (unless it's a HAMO product in which case you just need a decoder).

Looking online, your friend probably has a mechanical reverser so will have slightly more difficulty than my 3320 (I just pull out the Delta decoder and the 21MTC carrier clips into the same spot) but it shouldn't be the end of the world.

If you're concerned, I'm sure Trains4U over in Peterborough would be willing to do this work - as long as you lent them some C-Track to test it on!

Edit:
I should probably ask - do you mean 3-rail DCC/mfx or the full 2-rail DCC for use with Hornby etc?

If the latter - it's a nightmare. I'm about 3/4 of the way through a 3320 - I've done the 2-rail conversion and the motor upgrade, but I haven't gotten a decoder yet. No-one has stock of the drop-in HAMO conversion parts, nor are there aftermarket replicas, and while the electrical pickups were thankfully easy to find - wheelsets are not. I had to disassemble and file down Lima replacement wheels to properly fit the original axles; needing a bunch of tools and vast effort.

So if your friend was thinking to do that - gently discourage them.