This might work better if you form a more detailed question.
Ardour is free. Reaper can be used for free but also allows users to purchase licenses. You're using the same version whether you pay or not.
Audacity is free but is generally considered to be an audio editor rather than a full audio mixing environment. It has its uses.
There could be loads of reasons why you'd want to pay for a DAW. For example, you may simply not like a free one like Ardour. You may find it hard to get to grips with but trying a different one, you may find it much more in step with you, I don't know, there's plenty of reasons ! On the other hand a free one might be exactly what you need.
I have been using Linux Mint 20 for a couple of months and as an OS, I love it but not everything free is wonderful. For 5 months before it I used Zorin OS 15 and that was a pain in the patootie simply because of screen resolution issues.
I haven't used Ardour because I use a standalone DAW and for any virtual instruments I might use, I still use Windows XP. When I was with Windows, I used Audacity and still do but I'd never use it for recording unless I was literally only doing two track stuff. Like Steenamaroo says, it's more of an audio editor which is how I've always used it. As a matter of fact, it's great for editing songs from one's music collection, you know, sticking in a repeat chorus that you love or editing out a section you don't want or joining two separate songs together seamlessly and stuff like that.
Although I've never tried it on Linux, Tracktion Free runs on that platform and 'they' have tested it on Ubuntu 18.04 64-bit It's free, so it won't cost your friend to find out whether it'll work for them. FWIW, I'm using this DAW under Windows and it does everything I need. The product page (with a link to tutorials) is at Waveform Free | digital audio workstation band editing software - Tracktion. HTH