Since R* has blocked it so it's not possible for you to mod a car yourself and give it to a friend there is now a way around it.
Things you need: Friend, Car He Wants Modded & a Secondary Car (Doesn't Matter What Kind) & Make Sure Doors Are Unlocked So He Can Get In It
1.Take The Car Your Friend Wants Modded To LSC.
2. Drive Into LSC & Mod The Car To His Liking Once It's Modded Exit LSC. (Do Not Add Tracker or Insurance)
3. Open The Doors To LSC For Them By Driving The Second Car Up To LSC To Get The Doors To Open (Do not drive in)
4. Have the other player drive the modded car into LSC.
5. Once He Is Inside LSC, He Needs To Pause The Game & Go To The Game Store, Let It Load & Then Back Out Of It
6. Once He Exits The Game Store, He Will Spawn Outside He Can Now Drive The Car Into LSC & Add a Tracker & Insurance To The Car