As usual, we have nowhere near enough information to answer thoroughly.
The answer is not simple when dealing with a RE asset. While it can be a permissible investment (so long as we don't have a prohibited transaction involved, which I often see is the situation), it causes its own problems. For example, are there other employees in the plan? Is it participant directed or pooled? There can be BRF (benefits, rights and features) issues depending on whether all participants can buy into it or whether all participants are automatically part of it (as in a pooled trust). Then, you have the problem of annual valuation; expect to have to spend a significant amount of money every year to get professional appraisals that would stand up to an IRS audit (that means, not what 3 real estate agents think it might be worth). Also, if it is a really good investment, the client is turning what should be capital gain property into ordinary income property, which could be giving up a substantial tax benefit. Because the plan is not an active developer of real estate as a business, it should still be a passive investment. But depending on the "deal" with the developer, it can lead to potential UBTI, which I always tell clients that becomes their CPAs problem (not mine) since they will have to file TAXABLE income tax returns for the plan if there is enough UBTI.
There are lots of reasons for NOT doing this; more reasons for NOT doing it than doing it.