I'm sure I'd done this back when I didn't have a real recording rig. I can attest to it being safe. You have nothing to worry about that.Anyone know if this works or if it's safe?
Now, there is one possible issue. I've debated this with some people, but I've always been taught to NEVER unplug a tube amp from its cabinet when it's on. So, I'm going to stick with that logic. I'm not sure how the newer Marshalls are doing their effects loop. It's possible that all the signal is going there and if you go line out, then you will completely mute your amp.
It's also possible that the amp will still make noise. Give it hell and find out.
The signal should be line level or somewhere in that ball park. You may need to boost it a little or you may need to use a pad.
The effects loop comes in between the preamp stage and the power amp stage. So any gain added by the preamp tubes (your gain knob)is going to be there as well as your tone controls. Any of the effects of speakers and power tubes are gone. This is probably going to result in tones you DO NOT like. I HATE direct guitar recording. I'm guessing your buddy's amp has some sort of "emulated" out that is probably toneless, but has the fizz knocked off the top to make it usable for someone who is VERY forgiving with their recorded guitar tone.If so, would it be just clear guitar tone or would it have any preamp effects?