I'm still puzzled as to why the male died over night ,
When u got the male, is he healthy?, if he is, how is hes stomach look like, Is he full, too full or slim? when u got him from the pet store? , if he's overfed and full and u move them (male) into a 10gal tank is not advisable when he had a full stomach, coz this will made him stressed out, this can also cause him to have stomach problems,
I have some experience with my bettas , that when they full and u moved or changed water for them they get sick,
Lol juz an idea, but truth, the male betta might died of other causes also like , what medical treatment u use on the new aquarium water? , because medications on the water can stress the fish bad when he's too full. But if hes not full , juz think back , is he healthy when u got him from store, even if u no see any disease on the fish , It might having some small parasite or other unnoticeable disease. And when there's a change of water , change of environment, the diseases symptom on the fish got flared up and triggered an immune response on the fish . When the fish cannot stand the new water , which if u also apply medications , I guess it will go to heaven lol.
I have seen some betta's of mine who's having some mild bacteria infections and cannot stand the change of water , well it died of apparent unexplained reason lol . Maybe the stress of changing water and the new medications for the water, coupled with the mild infection hes having , triggered an reaction to hes condition thus made him succumb
Anyway I am sorry for ur new friend there,