Vishwamitra & Indra were enemies?
Yes, but when?
Vishvamitra was a kshatriya king earlier. And later he did intense penance and acquired the titles of Rishi (Valmiki Ramayana 1.63.2).
And as described in your question here only Indra sent Menka. All this
is the conspiracy of the gods to deprive me of my great austerities',
Vishvamitra thought. (Valmiki Ramayan 1.63.10)
But later he perform rigid austerities. After consultations the gods and sages decided that this son of Kushika (Viswamitra) deserved the status of a Maharshi (Valmiki Ramayan 1.63.16). And Lord Bhrahma himself gave the title of Maharshi to Viswamitra (Valmiki Ramayana 1.63.18). Then, Viswamitra bowed to Brahma, the Grandsire of all the worlds with folded hands and said:
O Adorable one, if you have conferred the incomparable title of 'Maharshi' earned by my auspicious deeds, I feel I have conquered the senses.
Then Brahma said to him, 'You have not conquered the senses yet. O Tiger among
ascetics, strive still.'
Then Viswamitra further does intense penance
O Best of Raghus dynasty (Rama) when the thousandyearold vow was over (Viswamitra) the great practitioner of vows began to partake cooked rice. At this moment Indra assumed the guise of a brahmin and asked him for the rice (he was going to eat). (Valmiki Ramayana)
This mighty, adorable ascetic having adopted a vow of silence decided
to give all the cooked rice to the Brahmin. The brahmin finished all
the rice. Viswamitra went without food but did not utter a single word
to the brahmin. Thereafter the distinguished ascetic practised
austerities for another period of thousand years, his breath
suspended. (Valmiki Ramayana 1.65.7)
i.e. He concurred sense, no anger no enmity.
Thereupon devatas along with gandharvas, pannagas, uragas, and rakshasas, stood bewildered by his ascetic energy. Their own glory was dimmed in his lustre". Thereafter, with Brahma in the forefront, the gods spoke to Viswamitra in pleasing words: Welcome, O Brahmarshi. (Valmiki Ramayana 1.65.9/1.65.19)
Then Viswamitra replied
If brahminhood and long life have been acquired by me by your grace,
Omkara, Vashatkara and the Vedas shall accept me as Brahmarshi. (I
shall acquire the right to interpret the Vedas and to perform
sacrifices.) (Valmiki Ramayana 1.65.22)
Virtuous Viswamitra also, having attained excellent brahminhood, worshipped Brahmrshi Vasishta who was the best of those who recite hymns. (Valmiki Ramayana 1.65.26)
And thats the point, after becoming Brahmrshi he even forgot his enmity with Vasishta (who was his greatest enemy, they fought a battle earlier, he went against Vasishta for Trishanku etc...). Similarly after becoming Brahmrshi he forgot his enmity with Indra and Devas.
Hope this answers your question...