If they are not sure but still want to try just to start up their journey then spending small amount to start is nicest thing they could able to do.
But if they are not really so sure about their decision to invest with bitcoin best if they listen to their doubts and just start to gather more information then invest if they are truly ready to take the risk and apply those things they learn.
With either big or small investment capital, newbies can start with small part of their capital, practice DCA for their investment. It's good method to invest successfully and also helpful for them to gain experience with time in the market. If they make mistakes, starting will small capital won't cause them big losses.
Fortunately with DCA strategy for investment, if they practice it properly, there will be no wrong entry, because their entries can be low or high at different investment rounds, but with time in long term, their entry price will be averaged.
Invest small capital, do it regularly, stay in the market for a long time, will help them to understand it market more. It's foundation for their investment success in future.