Sonja,
whether the couplers will be compatible will depend upon when & by who the Lionel rolling stock was produced by.
Early  Lionel HO was produced under license by Rivarossi and may have a horn hook type coupler.
Later production in the U.S. by Lionel should have a knuckle type coupler  that should be compatable with the Athern Genesis locomotive couplers.
I spoke with Athern and they said that most knuckle couplers would work.
If your acquaintence has rolling stock with the hook & horn type, it would be easy for them to convert over to knuckle couplers.  If not on all, making a transition car with a knuckle coupler on one end and leaving the hook & horn on the other would enable them to hook up to the Athern Locomotive and pull the rest of their cars. If they plan on mixing rolling stock, change over to knuckle couplers would be best.
If the other brand rolling stock has knuckle couplers, they should work with the Athern couplers.

Happy Rails!
Dave

Just realized you answered the question about the other brands already Thank you!!

[djacobsen  quote]Early  Lionel HO was produced under license by Rivarossi and may have a horn hook type coupler.
 ...snip...
If your acquaintance has rolling stock with the hook & horn type, it would be easy for them to convert over to knuckle couplers. [/quote]

I always had a little bit of "trouble" mounting the usual (KayDee brand in my case) knuckle couplers to Rivarossi equipment as their couplers are/were truck-mounted and I wanted to body mount them. This was HO.