Just recently had a mobile home removed and the water supply line had a shut-off valve on it and was shut off, but water came out when I opened valve. It was sticking up about 2 1/2 feet above ground. Called a plumber to cut it back and cap it off. He had a hard time with it and couldn't get it to stop leaking. It is black 3/4" pipe (marking on pipe says Hylan 100 psi 3/4").
Problem is after inserting a threaded adapter and screwing on metal cap, the clamps wouldn't tighten because the pipe was so hard, it was hard as a bone. He tried heating up the pipe to get get some flexibility into it so clamps would tighten, but no success. He was going to try to find something that would work better, but haven't heard from him yet, he was supposed to come today, but guess he got busy with other calls. Leak is not bad, maybe 15 drips per minute, which is probably 2-3 gallons a day, but it is tied into a well. Line from well runs to house and line to mobile home tied into it somewhere next to house.
Any idea what kind of pipe it is, plumber said it wasn't used anymore and it's next to impossible to find fittings for it. I would like to dig it back toward house and cap it off. Any ideas/suggestions are greatly appreciated.

I would guess Polyethylene or Polybutylene, although PolyB is usually gray , particularly if it softens with heat. You can buy barbed caps and use screw clamps.
https://tse1.mm.bing.net/th?id=OIP.Mglgz6MNrVVgeW5pjg3kvwEWEs&w=98&h=105&c=8&rs=1&qlt=90&dpr=1.25&pid=3.1&rm=2 (https://www.bing.com/images/search?q=barbed+cap+for+poly+pipe&id=7072AA510F305FDDE83E235581C492B0A7C5E096&FORM=IQFRBA)

I dug a little further back and found 04KA PE-3306 PS-11-69 marked on pipe. According to my research that would be polyethylene. As far as barbed caps go, would it be better to use brass or PE? The plumber put a plastic barb with a male threaded end in it and screwed on a metal cap, but couldn't get the leak stopped as clamps wouldn't tighten enough due to pipe being so hard. He said if he tightened clamps anymore it would strip them. I'm a novice and the plumber has been plumbing for over 50 years, and he has done a lot of work (good work) for me in the past, so I was surprised he had a problem. I want to make sure it is fixed properly and don't have a leak underground later on. Also don't want this temporary fix to fail and gush water when I'm not home and end up with a burned up well pump to boot!

I have used the plastic barbed fittings before without any issue. Sounds like cold weather is part of the problem. I would heat the end of the pipe so the PE is soft, drive the plug in and tighten hose clamps while the pipe is soft. You might be able to find a small muffler clamp at an auto supply store that could reduce that far. You could, in addition to the plug, drain some water out, plug the pipe and then bend the pipe back to crimp it and use a clamp to hold it back.

Update: Plumber finally got back out, he dug pipe back (about a foot) and located the tee where it branches off from well and house to the mobile home. He cut the pipe and removed it from the tee and he used what he called "soft roll" and inserted a steel barb with a cap on one end and slid it over the barb from the tee and put clamps on it (2 on the barb from tee and one up at the nipple. He's going to let it sit a while with the pressure on it and come back and check it and cover it back up. I have noticed a very minor leak from the soft roll where it goes onto nipple, but I think it can be stopped with another clamp, at least I hope so. He told me that if I ever had a problem anywhere else I would need to have a new supply line run from the well to the house, as everything is currently polyethylene from well to house.

I was scared to try to fix it myself, scared I'd make it worse and end up with an emergency, having a geyser gushing up in the yard. Does what he did sound like an appropriate and permanent fix and any ideas on stopping the minor leak if another clamp doesn't do the trick?

I appreciate all your advice!