Can't located friend on Find Friend app when he using 3/4G

Check this support document, and look under the area for "If you can't see a friend's location". That would troubleshoot on your phone, however the section above is what your friend might need to check to see why it isn't working for seeing them while on cellular. iCloud: Find My Friends troubleshooting

Then make sure your friend has cellular data turned on for the Find My Friends app.

Then your friend needs to troubleshoot their Find My Friends app. They are the one that is having the problem if you can locate them when on wi-fi. Set up and use Find My Friends - Apple Support

I know what it means to be on cellular. Please re-read my question.

What happens when you try to find your friend when they are on cellular? Have they turned off wi-fi? Even when not connected to wi-fi, it helps in location services.