Your friend should look at slowly increasing her weight so that her body frame can take the gain that might be added. If you say that she already has been trying to put on more by eating well, she may like to start off by a visit to the doctor to check if all is well with her. After getting an OK from the doctor, she could start on a strength training program under the guidance of a certified and experienced fitness trainer who can gradually tone her muscles and make her stronger. The weight may not be that noticeable but the training can help her in becoming able to endure life at later stages.