Lactation would be prolactin related, always.
Any androgen can elevate prolactin, even test. Depends on the user, and those levels do not tend to drop quickly, nor are they handled by SERM's or AI's.
"Delayed gyno" could have several factors. Normally its simply the user not respecting their drugs.
Primary cause. Stupidity. Using anything but a real, illegal, prescription drug as a PCT is not a PCT, even if users want to call it such. Estrogen fucks them straight up from the spike from the HPTA rebounding and being unbalanced when it does come back online. Or they have fake or underdosed drugs, either way youre still not doing it right.
Second most common. Not running their legitimate PCT drugs long enough. SERM's dont actually reduce estrogen at all, they block its effects, if during the PCT the SERM is blocking THE EFFECTS of your high estrogen level caused by your HPTA coming back online and attempting to stabilize, and you stop taking it...bam estrogen binds to receptors, you grow gyno, and anything inbetween. The SERM is really just covering your receptors till your body can stabalize itself on its own, this might take longer than "2 weeks" or some crap I see guys do with Prohormones, steroids are steroids, respect them.
The "tren" and "deca" prohormones especially are progestin based and can EASILY elevate prolactin as well as having direct progestin activity themselves, no surprises there.
Deca, tren, drol, alot of other actual steroids have pretty serious progestin activity and/or can elevate prolactin easily. Deca for example has a crazy long clearance time in the body, months. If the PCT is only 4 weeks, you still have deca in your system.
Even the most standard of all drugs, Test. If you get a longer half life version and dont give it enough time to clear your system, same issues.
And of course. ANY androgen can elevate prolactin levels, even test, even moderate doses, its just user dependent.
Only reliable method with steroids is being obsessively thorough.
I dont use long ester drugs, I always use an AI on cycle AND in the PCT, I always run long PCT's, and I stay on a low dose AI year round, regardless. I get regular blood work, and I use dopamine agonists when required.
Your buddy lactating from ONLY a Test E cycle. Possible. More likely he was running another drug that he didnt see fit to mention. Even a prohormone can mess stuff up if its the right one.
And like I said. If youre simply unlucky, any androgen can elevate prolactin, which SERM's and AI's do not fix, although they will limit the damage caused, only a dopamine agonist can correct the issue.