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.
This answer is as per the Kāraṇāgama (कारणागम), one of the 28 Śaivāgamas.
For a Dwija-Women, especially the Brahmiṇa-lady, it is preferred that the husband is the "initiator" for his wife, and thus her Guru.
For the Non-dvija women, the husband is not preferred to be an initiator because a non-Brahmiṇa as a guru is said to be unfavorable.
Quoting from Chapter 2 (dvitīyaḥ paṭalaḥ): kriyāpādaḥ
॥ द्वितीयः पटलः ॥
ब्राह्माणीनां हि दीक्षाया ग्रहणं भर्तृतो वरम् । भर्तर्यशक्ते ग्रहणमन्यतो वा विषिष्यते ॥ ५३ ॥
- It is good for brahmiṇa-woman to be initiated by the husband. If husband is incapable, then it is good to be done by others.
अब्राह्मणीनां युज्येत भर्तृतो ग्रहणं नहि । अब्राह्माणस्य पत्युवा नोपदेष्टुत्वमिष्यते ॥ ५४ ॥
- It is not good for a non-brahmiṇa woman to be initiated by her husband. Having been initiated by un-brahmiṇa husband is not honored.
पत्नया भर्तृगुरौ चैव भर्त्रा पत्नीगुरावपि । गुरुदृष्टिर्विधया स्यादन्यथा पततो धुवम् ॥ ५५ ॥
- Either a wife or a husband must keep reverence upon the husband - preacher or husband upon the wife - preacher respectively. If they don't do so respectively, they will be certainly deprived of that Vrat.
English Translation by R. M. Prabhulinga Shastri