Pardners are shotguns therefore a shotgun frame... SB1
An SB1 frame is fine for 357's and 44's and is sold as such from the factory but is not meant for a 223, 243, 270, etc.
If you look back into H&R's history you'll see that they used to sell complete rifles chambered in 22 Hornet, 22 Jet, 30-30, 357, 45-70 and probably some others on the iron frame (think SB1) and they worked fine.
Keep in mind these are all lower pressure rounds.
If your friend wants to fit one of the aforementioned chamberings on his Pardner frame (SB1) and stick to SAAMI specs. he should be fine... if he wants to go with a 223 or something else of that nature he's barking up the wrong tree.
Spanky