Mikey - "... I am much more interested in why he became involved in the first place..."
I don't know precisely why he got involved with them but I do know that he was suffering from depression at the time. Scientology, like all cults, prey on people who are desperate and/or depressed. They promise to 'cure' them of their particular malaise regardless of what it is. However, unlike most other cults, this entire organisation is predicated on relieving you of your money.
M - "... Did it never occur to your friend that the very basics of this newly invented religion were drivel ? Space ships arriving from outer space, many millions of years ago, etc, etc."
No. Because that's not what they tell the newbies. That kind of information is only revealed after many years of 'auditing' (are you aware of the process of 'auditing'?)* and several thousand pounds. My friend never reached anything like that level. I and another friend did bring this matter up with him once. He flatly refused to believe it and got quite angry with us for 'denigrating' his new found belief system and his new circle of 'friends'.
M- "... very foundation of the cult was ludicrous.......why could he not see that ?"
Good question. Why does anyone buy into tall tales and fairy stories? I think it's because there's something missing in their lives. Maybe it's love? Maybe it's purpose? I don't really know. But I do know that once you're in a cult, extricating oneself is extremely difficult. I'm just glad that he got out when he did and I'm glad to report that he now has a new partner and his life's back on track.
* further reading about Scientology:
http://www.xenu.net/cb-faq.html