Not concerning. In fact now recommended to reverse T2 diabetes and reduce impact of some other conditions. The evidence suggests the faster weight loss is motivating for further loss and losing 15kg in 8-12 weeks via VLCD is safe enough either through a managed plan or balanced food. If you'd like to support your friend in her goals, you can do this by avoiding meeting for meals or situations that may involve food, and doing things like offering her cakes (one won't hurt ... go on).
Its quite often friends who derail best attempts. They tend to think of someone as their fat friend and use them to feel better about themselves and don't like the readjustment to them not being fat. It's quite disturbing psychology - you'd not dare to consider a black/ asian/disabled friend in the same way.