I am looking to use my MileagePlus Miles for four business class tickets from IAD to Toulouse this summer. There is availability on United on the outbound but not on the return. A frequent flyer friend of mine, who flies Air Canada and Lufthansa, tells me that he sees availability on the outbound and return with Air Canada and also with Lufthansa when he is logged into both those sites. I only have miles with United; should I not be able to use my United MileagePlus Miles with Star Alliance partners and if so how?

Thanks.