The question of STI will come only if that lady was infected. if she was not infected, your friend may not have any infection.
If he wants your friend can visit a dermatologist and get examined to rule out simpler infections like fungal infections , herpes etc.
some infections like gonorrhea ,
HIV, herpes may take time to manifest. it would be prudent to do some tests like TPHA, HIV etc as per his suggestions. HIV infections have a window period of 6 months , so its better to do a repeat test at end of 6 months and if both these are negative, he has not got HIV.
In the window period, your friend must use condoms while having sex with his regular or any partner.
Next Steps
When one is using condoms, after ejaculation one must hold the condom rim against the base of the penis and slowly withdraw from the vagina. This will prevent such a fall of condom in the vagina