My friend has an old (pre-war) commercial Mauser sporter. It has the typical fixed sling swivels. He wants to add a sling that can be removed relatively easily and without tools. I have a sling like this on my pre-war drilling, but it cost me a pretty penny. He's looking to spend very little (<$35) but those I'm aware of are twice that and up.

Please recommend something in his budget. He wants to use the sling to climb up into tree stands and then remove the sling so it won't be in the way once he's in the stand.

I guess he could use anything with a couple swivel eye snap hooks.

I use something like this on my 500. I sling it while wading with gear then take the sling off and stuff it in my pocket.

Israeli sling

Also i ran para cord thru the sling studs and tied a small loop to hook to.

Also check out the Frank Procter sling.  wotg sling

Maybe not the nicest option, but a leather tabbed SKS sling or Mosin Dog Collar sling could work.

SKS Sling