It’s unlikely that an already properly installed bootloader just breaks. The base is Sid, Debian Unstable.

Just because breakage doesn’t happen all the time, there is still a higher than average chance. Sid is Debian’s beta test branch, not a rolling release distribution. It just wasn’t the right choice for the lady at the repair cafe.

I was corrected that LMDE is not based on Sid. I redact that part of my comments. The experience I had installing LMDE on a lady’s laptop at a repair cafe was as described, though.