Where rollover relief has been claimed, that precludes a claim to entrepreneurs' relief on the gain that has been rolled over. There was no CGT on that part of the gain so there can clearly be no claim to ER.
As the property your client bought has been let for 10 years, that doesn't constitute a business for ER purposes so he will pay CGT at 18% or 28% or a combination of the two rates depending on his income in the tax year of disposal. It would have needed to be used in a business run by your client that ceased no more than 3 years ago to qualify for ER.
Take a look at HS275 here for information on entrepreneurs' relief.
I hope this helps but let me know if you have any furrther questions.
If the property had been owned by your client personally, he would probably have been able to make a discounted claim for ER to take account of the letting of part of the property. If the property had been owned by the company operating the business itself, he could have sold the building, formally liquidated the company and claimed ER on the capital distribution. As the property is owned by a third party investment company and that company is selling the property, any profit it makes on the sale will be subject to corporation tax. I doubt that HMRC would accept a claim for ER after a formal liquidation of the company and the capital distribution.
Take a look at HS290 here for information on how business asset rollover relief works. It can only be claimed by an individual trading or in a trading partnership or providing a personally owned asset to their company. The original gain not covered by rollover relief previously may have been eligible for an ER claim by your client personally, not if it was owned by the company.
The new gain in a rollover case is determined by deducting the rolled over gain from the purchase price of the new asset but I would have thought that the new property would have been exchanged for shares in the new company so it would be the shares that have the discounted cost.