When you get your monthly salary, you aren't getting round numbers. You aren't being paid $10.000 a month. Maybe you earn $1374/month. You didn't turn your job down because the pay isn't a nice round number. Why don't you apply the same logic to Bitcoin?
Exactly what to say, it should be invest of what you can afford to lose because maybe someday they will back and ask you why the value of bitcoin holding at are losing value and the amount should not affect to their monthly allowance and buying the whole number of bitcoin is too much expensive as of now. But as OP said, their relative is an investor and maybe they understand the risk upon investing in bitcoin. I hope they fully understand the whole concept of bitcoin investment so that they will not become frustrated when the market price drop.