The tense in your sentence is Present Simple. The evening is spent with friends not because of some other tense used in the phrase, but because it is in the passive voice. The passive voice requires the appropriate form of the verb 'to be' + past participle, and spent is the past participle of spend.

If you don't use a preposition (e.g. 'on') - i.e. if you say 'I spend time [ing verb]' - what you are saying is that you are 'spending time by [ing verb]'. For example, if I 'spend time reading books', I 'spend time by reading books'. If, however, you do use a preposition and say, 'I spend time on [ing verb]', you are 'using time' to [bare infinitive of ing verb] - so if you 'spend time on reading books', you are 'using time to read books'. Practically, these are saying the same thing, but there are few subtle differences in their thorough meaning. These subtleties, for example, can be exploited in literature texts. For instance, saying, 'he spent months studying physics' (unless in explicit context) is emphasising that he was studying physics out of his own interest and curiosity; 'he spent months on studying physics', on the other hand, may be hinting that he is dedicating some of his free time to studying physics and improving in that area for educational purposes.