It really depends on your view towards your friends company. You could try looking at it from a different perspective not from the “just because he or she is my best friend, I got to invest in this” . Try to look at the long term potential and what this company strives to achieve in the long term, could be anything you judge to be of worth.
Ultimately it depends what your ultimate purpose to invest and gain long term returns or to help your friend out.
Or in the other note, if you and your best friend share a strong mutual sense of honesty, trust, between one another and you see the company worth investing, by all means go for it.
What I am trying to say is one should not get pressurized to invest, but should do one's own research first and make your own judgement in doing so.
Feel free to agree or disagree with me as there is no right or wrong answer, could be different for everyone.
Hope this helps and provides you with a perspective from another individual :)