Is it safe to keep a regular couch outside?

I will never trust upholstery from a yard sale (which could have bugs and rotten filling in it), so in most cases, no. However, if it is under a roof or something, then maybe.

Depending on the climate you live in, yes you could. Especially if it was under a porch roof.

I would be more concerned with mice, than bugs.

It will be a disgusting mess within 18 months.

Just think of how heavy it will be when it is saturated and filled with bugs when you go to take it to the dump because it got all mildewy.

you are correct -- in my experience

Depends on your climate and what you mean by 'outside' and what the couch is made of . I live in a relatively damp cool climate and have had an upholstered chair on my porch for years. It's fine. If I left it where it could get rained on, it would be junk within 3 months even though it's all made of hardwood. It would be junk even faster if it had any particleboard in it.

If it's under a porch/patio roof and never got wet from rain or dew, it would be OK for a couple of years.

It will be ruined. Water will destroy it. There is outside furniture available that he should look at. If he takes his time and spends a little more, he can find durable outside furniture as comfortable as regular furniture. But, it is not the first stuff you see at Home Depot.

if it is a vinyl one yes ok

No, you can't keep a couch outside and expect it not to rot from the inside and outside and get full of mould.