Waldo,
De-stressing - Some snakes will eat from your hand 10 minutes after they are caught. Others may take some time.
If it is a female, and Gravid - Please instruct your friend to let her go. If she has a big litter and your friend is inexperienced (you said he is) The babies may prove to be more than your friend counted on.
Snap a photo and post it, several Garter Gurus could probably tell if she is a gravid female. (unlike many species, Garter females are of "Amazon woman" proportions compared to males.)
Personally, I prefer to capture babies or juvies - They are easier to get adjusted and most vulnerable from predation.
Kind regards,
Wayne A. Harvey

our critters
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1.2.9 Storeria dekayi Casper, Xena, Athena, & Kids
0.0.2 Thamnophis marcianus - Checkered Garter Snakes
0.0.1 Thamnophis sirtalis - common Garter - Princess
0.0.1 Nerodia sipedon - Water Snake - "Aqua"
0.1.10 Storeria occipitomaculata - Red Belly snakes
0.0.1 Amelanistic Corn Snake "CY" Juvinile CB
0.1.0. Pueblan Milk snake "Oreo" adult CB
1.0.0. ASIAN GREEN SNAKE 3' WC Cyclophiops major
0.0.1. Savannah Monitor "CHOMPER" CB
0.1.0. Green Anole "Crystal" WC
1.0.0. K9 "ACE" Black Cockapoo
1.2.0. Feline"Felix" "Kaja" & "Silver"
0.1.0. calico RAT
2.4.?? Mice - Feeder farm - Crickets / fish
More herps than I could ever list out back on the land. 
