It is not okay to mix two dissimilar formulas. If you purchase an aftermarket brand formulated BMW’s HT-12 coolant, you’re safe. Otherwise, you’d have to flush the system. Which entails emptying then refilling the coolant system. Completely not necessary in this situation.

Your ‘friend’ is about to purchase a blue coolant and add it to green coolant. Though HT-12 is backwards compatible with Peak’s ‘European’ mix, since the engine is not overheating, you and your friend would be just as well to wait on the nearest BMW parts counter to open (or Amazon) and get a gallon of HT-12. (it must be mixed 50/50 with distilled water. The yield is 2 gallons of coolant. Meaning, you’ll have plenty for future use and now, need a spot to store it as well something to store it in.

Another option to get your friend immediate peace of mind is top the system with distilled water. You can score a gallon for $1 at a grocer. It likely won’t take much as the low level sensor triggers while there’s still adequate amounts of coolant in the system.