My gf has really bad burns on her body which cream/tissue oil can be recommended to heal her?

Proceed cautiously: Before you do that, use HER palm sizes to help estimate the region of the burn. 1 palm is roughly 1% body surface area. If she is roughly in the 15% range or she has very high degree/deep burns, see a doctor immediately. She may need specialized treatment or admission to a burn unit. https://www.emedicinehealth.com/burn_percentage_in_adults_rule_of_nines/article_em.ht

Need to determine the depth of burn wounds. Médical attention will be required to avoid infection. Please consult with A physician with burn care experience.

After thorough assessment of the burn, you could consider the use of aloe. It is important to stay well hydrated.