In terms of the drug screening, your friend does not have to worry about being tested for steroids, however the background investigations performed by FLE agencies is extensive (they will speak with people he hasn’t talked to in 10 years!). If at any point they believe that he has been deceptive in his application, he will be out of consideration for the position and blacklisted for any other FLE job. Additionally, steroid usage is an automatic disqualifier for the job, so he should not disclose it in the application.
To make a long story short, to get past this portion of the application process your friend needs to 1) not disclose any steroid usage in the application or to his background investigator (despite what they say, and they will lie to try to get him to admit to it if they suspect steroid usage), 2) Pass the polygraph - and they will ask about previous drug usage, including steroids, and 3) Not be ratted out by whoever the background investigator happens to contact (and it won’t just be the names that he submits, they will ask those people who else knows the applicant for more interviews).
Good luck to your friend.