I'd get in there and see what's up. If it's the lifters, I'd replace ALL four (4) lifters now. Less than a hundred for parts (HD stock lifters), and time. If you add (or if it has) adjustable push-rods, it's a pretty easy job...otherwise, I think you'll have to pull the heads. I'd spend the $150 on adjustables, and 80 or 90 on the lifters to not have to worry about one of the lifters collapsing. That'd screw your dad's day up pretty good. But first...make SURE it's lifter noise, and not something else. If you've got a mechanic friend, possibly ask them to help you sort it out.
Does it 'rattle' at start up then quiet down right away? If so, that's a classic hydraulic lifter noise. If it doesn't rattle at startup, I would tend to think it could be something other than a lifter. Harley's make so damn much noise, it's hard to tell one 'tick' from another, for me. Stuff going on in the back of the bike 'travels', and can sound like it's coming from the front. So make real sure about what's 'ticking' before you commit to anything.
If it IS a lifter, and doesn't rattle at startup but does when the engine's warm, I'd get those puppies out of there, and replace them NOW. That would mean one or more are collapsing a bit when they're fully pressured-up--a clear indication one or more are TOAST. Why it would only do it at a certain RPM is mysterious. But that's why you need to do some further investigation.
Ask your dad not to ride the bike until you get it sorted out.
With the engine warmed up, the lifters should NEVER 'rattle', in my opinion.
Since we're talking about lifters, do some searches on the Harley stock lifters. They used to be highly regarded. But I read a couple of threads a few months ago that called their reliability into question. I haven't seen any more though, so maybe there was nothing to it...
Also checked rocker arm end play, all were at .012" - .014" so I added a .010" shim on the non thrust side. Valvetrain is quiet as a mouse. Local indie had shovel head shims in .007", .010" and .015" sizes.
Sorry everybody got busy. He is runnin 20w-50 Mobil1 synthetic oil. I believe it has 14,000 or so miles. The noise is when it is warm and cold at that certain rpm. I will let him know the suggestions. Thanks!