There is nothing wrong in buying your friends property. What may be wrong is, if your friend asked for help and u refuse to help instead offer the choice of buying his property. Come to think of it most people hardly look at the other side of helping their friends. what if u help him with money and he refuse to pay back, that may compromise your friendship.
I always have one rule for my friends (unwritten and unspoken rule though) if i can't trust you with money there is a limit to our friendship.

I think there is 100 and more things wrong in buying a ''best friend '' property.
Money sometimes create problems in between friendship, but as a closed and best friends, u guys must have gone through thick and easy times together, they must have known each other limits.
Practical example, assuming best friend is in need of money to do something urgent, and at that moment he or she is broke, comes to you, swallowing his/her pride that i have some of my properties for sale e.g. refrigerator, cooker, home theatre set e.tc. so as to be able to get money to help himsslef out of the mess he his.
what do you do, start buying the properties, or rather help in looking for an alternative, maybe to laon him some but not all and ask him to look for others somewhere else, if he sell his property to an unknown person to you without your knowledge fine, so be it.
On the other hand the problem encounter is getting the money back, as a true friend indeed , he/she should return the money imediately they are done, but in most cases they will capitalise on the fact that after all its my friend money and will not want to return it back.
A similar example i encounter is this, i have to give out half of my salary back in Nigeria to a friend who is desperately in need, but as today i have not gotten my money back.
The rule i set formyself is, i only loan people money or assist friends if i know i can spare the money.
because immediately the money is off my hand i assume is as good as gone.
As a best friend i will be willing to do anything to help he/she out of the mess they are in financially without buying properties and putting my mind off the money.
it can be paid back or not.
this yoruba adage says (owo lo ba oju ore je)
meaning money breaks firendship.