thank you for your query. the half life of THC
is highly variable among individuals,depending upon Age, duration of use, quantity, purity of the substance, method of intake, body weight, body fat content, associated medical complications or drug or medication use if any. Also, please note further that the effects/ symptoms of marijuana may wear off relatively early, while it may take much more time for the body to excrete the chemical it completely through urine etc..
Complete elimination of THC may take about 7 to 30 days. however, if your friend has less body fat, you can expect it to be excreted early.
hope i've been able to satisfy your query. please don't hesitate to contact me for any further clarifications.