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.

Hold your horses! If it's for your setup; that's an MS2 on 3-rail C-Track, right? So no need for the nightmarish wheel replacement. ;)

The motor parts basically just bolt in, and for basic motor drive without lighting there are (I think), six solder joints - and the PCB end is pre-soldered for you. The fiddliest part is installing the brushes, it's not awful.

I would work out what kind of motor you have (so you know what replacement kit you would need) and evaluate costs. Because the last thing I need is more temptations in Waterside!

A good option if you don't want to replace the motor is the 60906 decoder.

The 60906 is basically a modern alternative to the 6080 decoder (or delta). Even if the performance is unregulated, it uses a PWM regulation to control the motor, so you get better driving performance e.g. at slow speed. My primary choice is e.g 60941 and e.g. 60977 or 60972 decoder, however, I have used the 60906 on a select few locomotives for nostalgia and/or cost reasons.