Is it allowed for a person to be a guru for his own wife? Does any scripture disallows it?
According to the Tantras it is usually not ok for the husband to initiate his wife. But if the husband is Siddha Mantra (mastered a Mantra) then he can.
The Brihat TantrasArah quotes the following verses from the Rudra YAmala Tantram in this context:
Na patnim dikshayed bhartA na pitA dikshayeth sutAm |
Na putrancha tathA bhrAtA brAtaram nacha dikshayet ||
Siddha mantro yadi pati stadA patnim sa dikshayet |
Shakti tvena varArohe na cha sA putrikA bhavet ||
In Rudra YAmala Lord Shiva says to Goddess PArvati:
A husband must not initiate his wife; a father his son or daughter; a brother a brother. However, if the husband is Siddha Mantra, he can initiate his wife but then he can not behave with her like a daughter (because for a Guru his disciples are like his children) like he does with his other disciples.
So, it is not totally disallowed. A Siddha Guru can initiate his wife but his behavior towards her should be like a husband and not like a father.
Now, if we talk about the Smritis (like Manu Smriti etc), then according to those texts, a wife is not qualified to be initiated (here initiation means the Vedic initiation). So, the question itself does not arise.
But Smritis say for a wife, his husband is her Guru and her stay in his house is the Gurukula VAsa and etc.
Manu Smriti 2.67. The nuptial ceremony is stated to be the Vedic sacrament for women (and to be equal to the initiation), serving the husband (equivalent to) the residence in (the house of the) teacher, and the household duties (the same) as the (daily) worship of the sacred fire.