Is the fact that each of a group of worshippers performs his prayers individually sufficient evidence to judge them as non-Muslims, and is it permissible to return the greeting of Salaam to those who offer it to us?

The default is to assume the best of a Muslim. What was mentioned about him praying alone is not enough to judge him as belonging to the Makarimah or Isma'iliyya sects. One should focus on da'wah (inviting to Islam) and giving advice. So, he should advise the manager to pray with the congregation. If his speech indicates his divergence from Ahl al-Sunnah, then his invitation should be with wisdom and good admonition. Sheikh Ibn Uthaymeen was asked about greeting Isma'ilis, and he replied: "Yes, return their greeting. Deal with them as they deal with you, and invite them to Allah Almighty."