A friend has had 2 AZ vaccinations and a Moderna booster, the latter 5 months ago. She had been going to get the 2nd Moderna booster at the same private hospital.The opportunity has come up to get a free Pfizer booster.Is there any issue? If there isn't, is there at least a preferred option to get a second (paid for) Moderna jab?

Get free pfizer, for a change. 
This private moderna might come handy later on, possibly as a second  generation, twicked for new strains. 
Or re-sold, given to somebody, who needs one, but unable to get any