A lot depends on the fund. Costs, equity creep in managed funds, and re-balancing too soon are a few of the dangers. I don't trust managed fund families to not use them to hide their mistakes.
The nice thing about Vanguard Target 2050 fund is that it is made up of:
63% Vanguard Total Stock Market Index Fund Investor Shares
27% Vanguard Total International Stock Index Fund Investor Shares
10% Vanguard Total Bond Market II Index Fund Investor Shares†
By using ONLY Vanguard index funds, costs are kept down, and yet there is the desired diversification. He can always change the target date periodically if desired.