You seem to be suggesting the 'upping' of the dishonesty from the not mentioning of a boyfriend in the UK when applying for a visit visa to totally changing the story of their relationship if they ever apply for a fiancee/spouse visa. If the ECO works that out (not everyone is a confident liar when being grilled in an interview...) that could result in a ban.
To be honest in this case the ECO may well suspect something is up and look into any application in more detail anyway. Asking for a visa for 2 months, then staying almost 6, and then applying for a visa (2 year one..) immediately suggests there is something in the UK worth returning for (usually work or relationship).