When you think about it, Animal insulin was the first insulin available in 1921, where has the human based insulin’s have only around something like 20 years… So one would assume that animal insulin is well tried and tested in over 80 years of use treating diabetes…
There are a few diabetics that can not tolerate the human version of insulin, so use animal insulin, and there are properly a lot more diabetics who have difficulties controlling there diabetes that might well benefit from transferring to animal insulin’s to see if this improves there control.
The only real disadvantage with animal insulin, is that you have to inject it around 30 minutes before you eat as it takes time to start to work in the system, which can be a disadvantage when you are not sure how many carbs that you either want to or going to have for your meal..
So if you look at it based on a proven track record, the I would say that animal insulin is well proven to be safe, I think that 80 years down the line any major problems with it would have been identified a long time ago… If you friend is happy and doing well there is no need to be worried or concerned about how safe the animal insulin is, you can assume that it is safe to use and well proven..