As has already been stated, it doesn't matter what 2G/3G/4G mobile network you are on, streaming has been disabled for mobile data. This is apparently due to rights restrictions, despite Sky Go not being restricted in the same way.
The app won't stream over 4G. Well actually, if you have 2 phones, hotspot (tether) one then connect the other phone to that hotspot, the app thinks it's connected by Wi-Fi, even though the data is coming via 3G/4G.
Either your friend us doing this, or is a BT direct subscriber, and therefore has access to the BT app. Sadly VM subscribers to BT Sport can't currently use their apps.