She should join some tour activities such as taking surfing lessons, snorkeling tours, sailing, etc. That way, she won't be totally alone and may be adopted by someone in the groups. December is also whale watching season in Hawaii.
She might have a little trouble driving to Hana (Maui) on her own, as that is a very long day. And without someone driving, it is hard to look at the scenery and drive at the same time, especially if she is not used to difficult drives - it is harder than Hwy 1 in CA.
Big Island is also good for a loner like you said. If she thinks her lonely romatic times can really get to her, maybe try avoiding the romantic places, such as sunset watching on the beaches, etc. It doesn't bother me a bit - maybe I am not that romantic a person, HaHa!! :-)))
By the way, I have been a solo traveller since last Monday and this lonely trip won't end till this Friday, both for work and play. It's a bit long. So talking about a lonely unromantic traveller, I'm right in the middle of it!
Last night, I went to the world's largest hot spring pool in Glenwood Springs, CO. Romantic couples were all around me in the pool. So I ended up burying my head in 94 degree water and swimming most of the time while there. That was one of my best moments on this trip! :-))